The present invention related to a powder classifying device that classifies powder having a particle size distribution according to one or more desired particle sizes (classification points), and more particularly, to a powder classifying device that can classify powder preferably having sizes equal to or smaller than about several micrometers with a high degree of accuracy making use of a balance between centrifugal force given to the powder by circulating air streams and drag.
For example, a powder classifier as disclosed in JP 06-83818 B is known in which a powder input port is provided in the center of the upper surface, a powder path is formed along the conical surface spreading from the vertex of a cone provided right below the powder input port, the lower end of the powder path is located in substantially the center of a plurality of guide vanes arranged to extend at a predetermined angle from the circumferential edge toward the axial center, a center opening connected to an exhaust pipe is provided in the axial center below the cone and an air inlet is provided on the outer side periphery of the guide vanes, the guide vanes are divided into two stages including upper and lower stages by a partition plate, the powder path is opened between the guide vanes at the upper stage adjacent to each other, exhaust air discharged from the exhaust pipe causes air introduced through the air inlet to form a circulating stream when the air passes among the guide vanes, and powder is classified according to the balance between centrifugal force given by the circulating stream to the powder which falls from the powder path to spaces between the guide vanes, and drag.
Having the configuration described above, the powder classifier increases the powder processing ability while ensuring the circulating motion of powder owing to the circulating stream, and thus achieves the effects of uniform powder acceleration and an improvement in classification accuracy.
Air is flowed toward the guide vanes from the periphery to the center thereof, i.e., along the radial direction, then its direction of flow is changed by the guide vanes. Therefore, the guide vanes can reliably change the direction in which the air is flowed and therefore the classification point.
Moreover, since the guide vanes are provided in the two upper and lower stages, powder having been charged into the spaces between the guide vanes is guided to a classifying zone integrally with air streams without being precipitated. Therefore the powder is classified in a uniformly mixed state while improving the classification accuracy.
JP 08-57424 A discloses a material supply device suitably applied to a device (pneumatic classifier) in which a material supply cylinder is provided in the upper part of a classifying chamber, a material is supplied into the material supply cylinder and circulated, causing the material to move downward and be introduced through a supply hole provided on a lower outer periphery of the material supply cylinder into the classifying chamber where it is classified. In the material supply device, a plurality of guide vanes inclined in a direction of circulation of the material are annularly arranged on the outer periphery of the material supply cylinder and secondary air inlet passages are provided between the guide vanes adjacent to each other.
With the material supply device, when the material is supplied into the material supply cylinder and circulated, secondary air is introduced into the material supply cylinder through the secondary air inlet passages between the guide vanes. Consequently, a dispersion force can be imparted to the material and a semi-free vortex can be formed inside the material supply cylinder, which enables the powder material to be supplied in a dispersed state into the classifying chamber at high speed.
JP 11-138103 A discloses a pneumatic classifier which is similar to the device disclosed in JP 08-57424 A. A classifying cover and a classifying plate are respectively provided in upper and lower parts of the pneumatic classifier in a such a conical shape that the lower surface of the classifying cover and the upper surface of the classifying plate have larger heights toward the centers thereof. A plurality of louvers (which are similar to the guide vanes provided in the device disclosed in. JP 08-57424 A) are annularly arranged on the outer periphery of a classifying chamber formed between the conical lower surface and the conical upper surface, an inlet passage for secondary air is provided between adjacent louvers, powder supplied into the classifying chamber is circulated at high speed and centrifuged into fine particles and coarse particles, the fine particles are discharged from a fine particle discharge cylinder connected to the center portion of the classifying plate, whereas the coarse particles are discharged from a coarse particle discharge port formed on the outer periphery of the classifying plate. In this device, “the inclination angle of the conical lower surface of the classifying cover is set larger than that of the conical upper surface of the classifying plate”.
In recent years, along with the advance of technologies, fine particles having a narrow particle size distribution are more often required.
Of the powder classifier disclosed in JP 06-83818 B, the pneumatic classifier employing the material supply device disclosed in JP 08-57424 A, and the pneumatic classifier disclosed in JP 11-138103 A, the pneumatic classifier disclosed in JP 11-138103 A can be applied to the above-mentioned purpose of obtaining fine particles having a narrow particle size distribution.
However, the conventional powder classifier and the pneumatic classifiers include a large conical material supply unit or classifying unit. Therefore, the structure (manufacturing process) of the devices is complicated. When powder having high adhesion properties or particulates on the order of microns (equal to or smaller than about several micrometers) or sub-microns are classified, a satisfactory result cannot be obtained in terms of classification accuracy and operability (or particle size controllability).